29 January, 1998
Hello from the Southern Ocean! We're making all kinds of neat maps here around Cape Adare. We have been running the ship back and forth . . . from the coast to the edge of the continental shelf (the shelf break) and back . . . as well as running up and down parallel to the coast. The goal is to get an accurate picture of what the land looks like beneath the water. Since the glaciers and their extra large icebergs had a definite effect on pushing the seafloor sediments around, we hope to look at the map and figure out what the glaciers did 20,000 years ago. It's really neat that we can see "iceberg furrows" in the maps! They are large grooves that were carved out by the bottoms of the extra large icebergs. We know these grooves are caused by icebergs and not by glaciers because they move in a wide variety of directions. The glaciers tended to move in one general direction.
Sometimes we have to change our direction because of all the ice in our path. There are several types of ice in Antarctica. Most of what we are seeing is called pack ice. Pack ice is made up of various sizes of ice pieces that are floating with the currents. Loose or open pack is when the pieces of ice are spread apart with water between them. Close pack is when the pieces of ice are pretty close together and you can't see much water at all. Pack ice can be found around the continent of Antarctica at all times of the year.
Sea ice is the name given to all the ice that freezes from the sea water. It can grow to a thickness of many feet. During the winter, the amount of ice around the continent grows substantially. Antarctic sea ice cover varies from a minimum of four million square kilometers in February to a maximum of 20 million square kilometers in September. As you can see, sea ice doesn't just happen all at once. As it gets colder in the Antarctic autumn, some of the open water develops needle-shaped crystals forming a slush. This is sometimes called frazil ice. If a film forms across the surface, it is called grease ice. Grease ice gives the water a matte appearance -- we were able to see some the other day when the water was especially calm. As the water continues to freeze into pieces, it is called pancake ice. Typically, pancake ice is made of pieces that are less that 20 feet across. It is called pancake ice because the edges are rounded as the waves jostle them against one another. As the ice continues to grow and thicken, the pancakes get bigger and bigger. In addition to the freezing of sea water, snow is added to the top of the ice to help increase it's size. If the pieces become larger than about 20 feet across, they are called cake ice. Eventually the ice forms one solid, floating layer which can reach a thickness of many feet.
The Nathaniel B. Palmer is built to break sea ice up to 3 feet thick at 3 knots. If we are breaking small pieces of ice, the ship actually runs up on the ice and the weight of the ship breaks the ice into pieces. In order to break heavy ice, the ship rams back and forth. There is also a way to make the ship roll from side to side if necessary. The trick is to break up enough ice that the boat can move through the water. For our research, however, we have been trying to avoid lots of ice because ice makes it difficult for our equipment to work properly. In fact, this morning the deep tow "fish" got tangled around a piece of ice and the captain had to move the ship carefully around an entire circle to get the "fish" free. We were all awed by his driving abilities! We haven't had to break any heavy sea ice since I've been aboard. When we go through pack ice, it sounds almost like a carwash inside the ship. The closer the pack, the louder it gets! I can just imagine the sound when they do have to ram back and forth!
All this talk of ice brings us to yesterday's question: What is meant by the term "fast ice?" Fast ice is a type of sea ice that occurs near the coast or in protected bays. In these areas, sometimes the sea ice becomes so large and spreads so far that it freezes to the land on more than one side. This makes it impossible for the ice to move -- in other words, the ice is held "fast" to the land. Fast ice tends to form in the area around McMurdo Station. The Coast Guard has two heavy-duty icebreakers (Polar Star and Polar Sea) that alternate years coming down to Antarctica to break the fast ice in McMurdo Sound.
I'm looking forward to tomorrow. We will be using a new type of instrument to gather data. It's called a CTD, and it is something that is commonly used by oceanographers. What do you suppose the letters "C," "T," and "D" represent? Keep your questions coming my way! Thanks for all the email!
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